Project Summary/Abstract Dental caries is the most common chronic infectious disease globally and is caused by the formation of acid- producing bacterial biofilms on the tooth surface, which demineralize and destroy the protective underlying enamel barrier. Although the efficacy of fluoride treatments (the contemporary standard in caries prevention) is well-documented, the current prevalence of the disease clearly illustrates that fluoride alone is insufficient to prevent caries in many situations. Therefore, increased understanding of disease pathogenesis and exploration of novel preventative strategies are objectives worthy of attention. Regardless of the microbial taxa involved, bacterial acid-tolerance is an indispensable factor in caries pathogenesis. The known caries pathogen Streptococcus mutans increases the proportion of unsaturated fatty acids (UFAs) in its plasma membrane in response to environmental acidification?an adaptation required for acid-tolerance and virulence. Preliminary data indicates that several other Gram-positive oral taxa, including the caries-associated species, Lactobacillus casei, modify their membranes in a similar manner in response to environmental acidification. This proposal addresses a number of currently unanswered questions raised by these observations. Aims 1 and 2 of the proposed research will determine the scope of this response to acid stress across the oral microbiome, in single taxa or in a community setting. Aim 3 of this proposal will elucidate how these UFAs are protective against acid- mediated damage. These aims will be accomplished using bioinformatics tools, basic molecular microbiology, an ecologically-relevant and complex in vitro oral biofilm model, and mass spectrometry/lipidomics. Successful completion of the proposed research will answer pertinent questions regarding caries pathogenesis in a multi- species setting and is likely to open the door to investigation of novel anti-caries therapeutics which, while targeting acidophiles, function regardless of the presence and abundance of S. mutans. The candidate, Dr. Jonathon Baker, has a longstanding interest in the microbiology of dental caries. Upon completion of the K99 (mentored) phase of this award, his goal is to become an independent PI at a leading research university, where he plans to continue research on the modifications that bacteria make to their membranes to combat environmental stresses, while leveraging acquired data to develop novel therapeutics. A funded K99/R00 proposal will allow Dr. Baker to develop skills necessary to both complete the proposed research (training in mass spectrometry/lipidomics) and subsequently become an independent research scientist (training in didactic lecturing, mentoring, and grantsmanship). Dr. Baker?s mentors and environment: Drs. Karen Nelson (J. Craig Venter Institute), Anna Edlund (J. Craig Venter Institute/UC San Diego), Pieter Dorrestein (UC San Diego), Victor Nizet (UC San Diego) and Robert Quivey, Jr. (University of Rochester School of Medicine and Dentistry) have been carefully selected to provide high-quality, diverse scientific and collegial support, as well as state-of-the-art facilities, to ensure successful completion of this research program and the proposed career development goals.